In a microprogrammed digital computer or data processor, microwords are read from an internal storage unit to provide the internal processor control actions which are needed to execute the processor instructions obtained from the user's software program. The microwords are read from the storage unit and set into a control register one at a time. Each microword consists of multiple binary bits, some or all of which may be organized into plural-bit control groups or fields. The control register drives a decoder mechanism which produces the elemental control point signals which, in turn, control the various data flow gates and data manipulation mechanisms located in the data processor. Each microword controls the internal operation of the data processor for one microword cycle. Several microwords are normally needed to execute each processor instruction. Some processor instructions require only a few microwords, while others require many microwords to complete their execution.
A good example of a microprogrammed data processor is described in U.S. Pat. No. 4,173,041, issued on Oct. 30, 1979 to T. J. Dvorak et al. As indicated in this Dvorak et al patent, the microwords may be located in a separate storage unit called a "control storage", or they may be located in a portion of the processor main storage unit which is set aside for the exclusive use of the microwords. For the case of separate control storage units, these units may be of either the read/write type or of the read only type. In smaller data processors, the current trend is to store the microwords in a so-called read only storage (ROS) unit. In any event, the microword storage units currently used are of the random access type and include the customary addressing circuitry for addressing any desired one of the storage locations in the storage units.